Sustained burning of fossil fuels over the last one and a half centuries has led to an increase in the atmospheric CO2 concentration by more than 45%, from about 280 to over 406 ppm (Earth System Research Laboratory, Global Monitoring Division, NOAA. https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html). As a result, the global temperature now exceeds +1° C. relative to the preindustrial era (Blunden, J. & Arndt, D. S. Eds. State of the climate in 2016. Bull. Amer. Meteor. Soc. 98, Si-5277, 2017). In order to meet the goals of the Paris Climate Agreement to limit global warming below 2° C. by the end of this century, an aggressive plan is needed for reducing fossil fuel emissions and gradual decarbonization of the global economy. However, recent studies indicate that might not be sufficient after all, and stabilizing the climate to an optimal level will likely require large-scale deployment of ‘negative emissions’ technologies (NETs), i.e., removing CO2 from ambient air (Hansen et al. Young people's burden: Requirement of negative CO2 emissions, Earth Syst. Dynan. 8, 577-616, 2017). Indeed, as the Earth is out of energy balance with the current atmospheric composition, more warming is expected even if fossil fuel emissions were to suddenly stop (Mauritsen, T. et al., Nature Climate Change DOI:10.1038/nclimate3357, 2017). Furthermore, it has been recently suggested that, ideally, the atmospheric CO2 concentration should be below 350 ppm, to bring the global temperature back within the optimum range of the pre-industrial Holocene period. Meeting this ambitious goal would require the removal of at least 550 Gt CO2 (550 billion tons of CO2) out of the atmosphere by the end of this century (Hansen et al., Ibid.).
Direct air capture (DAC) of carbon dioxide from ambient air by engineered chemical reactions represents a distinct category of NETs among other more “natural” approaches to negative emissions, including bioenergy with carbon capture and storage, afforestation and reforestation, and enhanced weathering of minerals. DAC has the advantage of relatively low land and water requirements, and it has been estimated it could remove up to 12 GtCO2/year (Psarras, P., et al., WIREs Energy Environ 6:e253, DOI: 10.1002/wene.253, 2017). However, living up to such great expectations requires sustained research efforts over the next few decades to improve the existing DAC technologies or develop completely new ones that are economical and can be deployed on a large scale.
The low concentration of CO2 in the air (˜400 ppm) and the inherently open nature of the DAC process impose some constraints on the type of sorbents that can be used. First of all, the CO2 binding has to be relatively strong and selective against other atmospheric components (especially water), which automatically disqualifies most physisorbent materials. Toxic and volatile liquid sorbents, such as amines, the workhorse of industrial CO2 scrubbing, are also undesirable due to the negative environmental impact expected from their large-scale deployment in open spaces. To date, there are two classes of sorbents that have been extensively investigated for DAC applications: aqueous alkaline sorbents (i.e., NaOH, KOH, Ca(OH)2) and porous solid-supported amines. The aqueous alkaline sorbents have the advantage of ready availability and relatively fast sorption kinetics, but are highly corrosive and the sorbent regeneration is energy intensive, requiring very high temperatures of ˜900° C. Solid-supported amines have lower regeneration energies and temperatures but tend to have slower sorption kinetics and their optimum performance requires maintaining a high surface area over multiple cycles and preventing water condensation in the pores (Wang, T., et al., Greenhouse Gas Sci. Technol. 6, 138-149, 2016). They also tend to chemically and thermally degrade over time, especially when heated in open air. A different approach based on anion-exchange resins acting as moisture-swing CO2 absorbents under mild conditions has been demonstrated, although the partial pressure of the released CO2 is relatively low, requiring additional concentration steps before storage (Wang, T., et al., Environ. Sci. Technol. 45, 6670-6675 (2011); Wang, T., et al., Phys. Chem. Chem. Phys. 15, 504-514, 2013). Thus, there remains a need for developing new technologies that combine the best attributes of liquid and solid sorbents, and that are energy efficient and cost-effective.